The Physics of Tsunamis

A tsunami is a series of ocean waves, sometimes called a wave train, generated by the sudden displacement of a large volume of seawater. The most common cause is an undersea earthquake along a subduction zone, where the abrupt movement of the sea floor lifts or drops the water above it. Underwater landslides, volcanic eruptions and, more rarely, asteroid impacts can also set off tsunamis. The term comes from Japanese words meaning harbour wave, and tsunamis are also referred to as seismic sea waves.

Tsunamis behave very differently from ordinary wind-driven waves. They have extremely long wavelengths, often tens or hundreds of kilometres, and carry energy through the entire depth of the water rather than only near the surface. In the deep open ocean the wave height may be only a metre or less, so it can pass beneath ships almost unnoticed, yet it travels very fast, comparable to the speed of a jet aircraft over the deepest water. The speed depends mainly on water depth, decreasing as the ocean becomes shallower.

As a tsunami approaches the coast and the water shallows, this slowing forces the energy of the long wave into a smaller volume. The wave train piles up, its height grows, and it can surge inland as a fast-moving wall or flood rather than a single curling breaker. Often the sea first withdraws dramatically before the largest waves arrive. Because the open-ocean signal is hard to detect by eye, modern warning systems rely on seismometers and pressure sensors on the sea floor to forecast arrival times for distant coasts.

Frequently asked questions

Why is a tsunami barely noticeable in the open ocean?

In deep water a tsunami has a very long wavelength but a small height, so the surface rises only slightly over many kilometres. The wave becomes dangerous only when it slows and grows in shallow coastal water.

What determines how fast a tsunami travels?

Tsunami speed depends mainly on water depth. The wave moves fastest over the deep ocean and slows as it enters shallower water near the coast.

Why does the sea sometimes pull back before a tsunami?

If the trough of the wave train reaches shore first, the water level drops and the sea retreats unusually far. This withdrawal is a recognised warning that a large wave may follow within minutes.